Physics in Action: Toss-and-Catch from Two Points of View

What does it mean when we say that motion in the horizontal direction is independent of motion in the vertical direction? Today's free Physics in Action demonstration: Toss-and-Catch from Two Points of View looks to answer that question. In the experiment, a cart is placed on a track so it can only move in a horizontal direction. Within the cart is a ball that can be ejected in the vertical direction by tripping a spring on the cart. We know that if the cart is not moving, the ball will land in the same location from which it was launched. However, where will the ball land if the cart is moving in the horizontal direction when it is launched? There are a few possibilities including:

1. The ball drops in the exact spot it was launched from, or2. The ball moves at the same horizontal speed as the cart and lands in the cart.